The invention relates generally to brake drums, and more particularly to an improved method of manufacturing a composite brake drum ring.
Most wheeled vehicles are provided with a brake system for selectively inhibiting the rotation of the wheels to slow the movement of the vehicle. One type of commonly known vehicle brake system includes a brake drum assembly provided at one or more of the vehicle wheels.
A drum brake assembly includes a hollow cylindrical brake drum secured to the wheel of the vehicle for rotation therewith. The drum includes a radially inner annular braking surface. The drum brake assembly further includes a brake shoe assembly having a pair of arced friction shoes secured to non-rotatable components of the vehicle and disposed in a spaced apart relationship with the drum braking surface. During vehicle braking, the friction shoes are moved into frictional engagement with the braking surface of the drum to inhibit rotation of the drum and its associated wheel.
Brake drums were typically made of gray iron because it provides the resultant drums with sufficient mechanical and physical properties such as hardness, strength, wear resistance, thermal conductivity, and the like. However cast iron is heavy which can be undesirable for use in a vehicle. In an effort to reduce the weight of vehicle components, composite brake drums have been developed which use steel in place of much of the heavier cast iron. Composite brake drums typically include a drum back for mounting the brake drum on a vehicle, and a composite drum ring connected to the drum back. The composite drum ring includes an annular steel band surrounding an annular cast metal insert. The braking surface is formed on the radially inner surface of the annular cast metal insert.
A known method of manufacturing the composite drum ring includes centrifugally casting the cast metal insert in the drum band. A drum band is provided which is preferably formed of steel in a conventional manner, although other suitable materials may be used. The band is formed in a conventional manner to predetermined dimensions. The band is rotated and molten metal, preferably gray iron, is delivered to the radially inner surface of the annular band. Centrifugal force evenly distributes the molten iron along the inside of the band. The band is rotated for another period of time to allow the iron to solidify. The braking surface is then typically formed on the cast iron insert in a known manner such as machining.
Alternatively, the cast metal insert may be statically cast in the band using conventional techniques. For example the band is positioned in a mold apparatus and molten metal, preferably gray iron, is introduced into a mold cavity defined between the mold and the radially inner surface of the band. The iron is cooled to solidification. The braking surface is subsequently formed in a known manner.
It is desirable that the circumference of the finished brake drum ring be round to within predetermined tolerances. Deviations from round, called roundness run out, which are larger than the predetermined tolerances result in the brake drum ring being "out-of-round". A brake drum having an out-of-round drum ring will be imbalanced as it rotates on the vehicle and may exhibit undesirable wear characteristics.
The conventional processes of forming a composite brake drum ring described above result in an excessive number of drum rings that are out-of-round. Additional processing steps are then required to "true" the drum ring thereby bringing it to within the desired tolerances for roundness. For example, it is known to provide the drum ring with additional cast iron which then must be removed to true the drum ring. Alternatively, steel weights are welded to the drum ring to balance the casting. Despite these measures, some of the composite brake drum rings cannot be trued and must be scraped.
It is desirable to provide a method of manufacturing composite brake drum rings which reduces the number of rings that are out-of-round. It is also desirable to reduce the number of composite drum rings that must be scraped.